IVUS is used to visualize the interior of blood vessels. Reflected ultrasound signals, usually generated by a transducer or transducer array located at the tip of a catheter that has been inserted into the blood vessel, are received, processed and analyzed to obtain IVUS images. The transducer transmits ultrasound signals, usually within the range of 10-50 MHz, within vessel lumen towards the vessel wall. The ultrasound waves are reflected from the tissue or from the boundaries between two substances with different refraction index, received in the catheter, where the varying pressure waves are converted to electrical signals which are then conducted to an external computer for processing. The reflected signals may be transformed into a pattern that represents a blood vessel segment, to produce a real-time image of a thin slice of the blood vessel surrounding the catheter tip. The 360° internal cross sectional image may be obtained by continuously rotating the transducer within the catheter or by using an array of ultrasound transducers aimed at the inner surface of the lumen. The catheter is pulled continuously along the blood vessel (Z-axis) to obtain images from different blood vessel segments.
IVUS may be used in all types of blood vessels, including but not limited to arteries, veins, and other peripheral vessels and in all parts of a body.
Generally, the transmitted and received ultrasound waves are initially translated to analog signals. These analog signals are amplified to provide optimal use of the dynamic range. The signal samplings may be delayed individually to focus the beam to a certain depth and direction. The signals are weighted and finally are summed up, in phase, to obtain a desired radiofrequency (RF) signal. The processed signals form a set of vectors comprising digital data. Each vector represents the ultrasonic response from a different angular sector of the vessel, i.e., a section of the blood vessel. The sampling from each vector and number of vectors, used to scan complete 360° cross section of the vessel, may depend on the type of IVUS system.
The vectors from one cross section form a two-dimensional array or matrix in Polar coordinates, i.e. I(r, θ). In this coordinate system, one axis corresponds to r, i.e., distance from transmitter to reflection segment, and the second axis corresponds to θ, which represents the angular position of reflection segment. Each element of the matrix I(ri, θj) represents the intensity of reflected ultrasound signal, which translates to tissue properties at a specific location.
The data in Polar coordinates are not usually transferred to a display because of the difficulties of interpretation by physicians. The data in Polar coordinates usually undergo several post-processing stages and are then transferred to Cartesian coordinates I (X, Y), where Xk=X(ri, θj) and Yl=Y(ri, θj). Such data, when displayed, represent the vessel cross section and can be analyzed by the physician. Images of vessel cross sections are acquired and displayed at a rate that depends on system type. Some systems acquire and display at about 30 images per second.
The blood vessel may be examined using IVUS by pulling the catheter back along the vessel (Z-axis), so successive series of images of corresponding vessel cross sections are displayed. The catheter is usually pulled back automatically at a constant speed (0.5-1 mm/sec), but also may be pulled manually, thus permitting vessel examination in three dimensions.
During slow catheter pullback, fast catheter motions occur relative to the blood vessel. These motions may occur in the transverse (XY) plane, for example, from shifting of the catheter relative to the blood vessel Z-axis. These motions may be seen as vessel rotation relative to the transducer, or as tilting of the transducer's 360° plane beam relative to the vessel (angulation), so that the XY plane is not perpendicular to Z-axis. Fast catheter motions also may arise due to backward and forward motion of the catheter along the Z-axis. Another source of fast motion is blood vessel compliance, i.e., changes of blood vessel volume due to blood pressure. Movements may be caused by, among others, heart beat, blood flow, vasomotion and other physiologically caused forces. Usually the vector of fast motion is a combination of all of these vectors. All relative motions are displayed as distortions or jittering of the IVUS image. This makes it difficult for the physician to accurately interpret the blood vessel morphology in IVUS dynamic display.
Current IVUS equipment and interpreter algorithms have no stabilization function that compensates for relative movements between catheter and the lumen. As a result the non-stabilized IVUS images may result in misdiagnosis, inaccurate interpretation of morphology and inaccurate 3D lumen reconstruction, which is widely used by physicians.
Where the non-stabilized behavior is a periodic function related to the cardiac cycle, the stabilization may be performed by synchronizing the image processing and pullback function with acquisition of ECG signals.
Morphological features of blood vessels can be divided into three categories: the lumen, where blood flows; the vessel layers, i.e., the tissue inside the blood vessel; and the exterior, i.e., tissue outside the vessel. In IVUS, the blood presents as rapidly changing speculars.
IVUS images are generally interpreted through analysis of separate images or frames. When the IVUS video is displayed dynamically, it is not always easy to separate the slowly changing, sometimes subtle morphological features of the blood vessel due to fast image changes.
Currently tissue characterization of a blood vessel may be performed by virtual histology. The results of IVUS data analysis may be correlated with histopathologic examination. Atherosclerotic coronary plaques are characterized in terms of classification trees. The different areas are assessed within the region of a target lesion using pre- and post-debulking data collection scans and predicted plaque composition is displayed as a color-coded tissue map.
The shape and size of the blood flow lumen is a very important diagnostic parameter. When this information is required, it may be determined manually by qualified person. The contour of the lumen is developed manually; the series of contours presents the lumen 3-D shape. Currently, software applications allow automatic extraction of the lumen for individual frame or image, but these may be inaccurate and time consuming and therefore may not be feasible for near real-time analysis. When either manual or automatic methods are applied to rapidly changing images, the analysis of the blood vessel lumen becomes difficult and inaccurate.
Current common practice is to perform repeated pullback examinations. For example, first a pullback examination may be performed to locate the area of interest (disease area), then the therapy such as PTCA, stenting, atherectomy etc is performed, and the pullback IVUS then repeated in order to assess the therapeutic results. The pre-treatment and post-treatment images or series of images are compared by manual matching of the images. Because of image instability, however, such matching is difficult. Furthermore, the orientation and visualization of the images may be changed, so identification of the anatomy may be time consuming and inaccurate.
Therefore, there is a need for a device and method for stabilizing IVUS images to improve the accuracy and diagnostic benefits and lessen the difficulty of IVUS interpretation.